Ball chair



FIG. 1 is a perspective view of a ball chair showing my new design;

FIG. 2 is a front elevational view thereof, the rear elevational view being identical;

FIG. 3 is a left side elevational view thereof, the right side elevational view being identical;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

CLAIM The ornamental design for a ball chair, as shown and described. 